1. Field of the Invention
The present invention relates to a recording ink which includes pigment particles serving as a coloring agent and dispersed in the ink.
2. Discussion of Related Art
Conventionally, an aqueous dye ink composition, in which a dye as a coloring agent is dissolved in water, is widely used as a recording ink for use in an ink jet printer. To produce the ink composition, various kinds of water-soluble dyes are dissolved in water or a water-soluble organic solvent, and various additives are added as needed to the solution. However, the aqueous dye ink composition thus obtained exhibits insufficient water fastness and lightfastness, leaving some room for improvements.
In view of the above, there has been developed an ink composition containing a pigment or pigments, which exhibit(s) higher water fastness and lightfastness than the dyes as described above. The ink composition is generally used in ordinary printing methods, other than ink jet printing, such as photogravure, flexo graphic printing, screen printing, typographic printing and offset printing, or used in ordinary printing or coating methods, such as spraying, brushing, and doctor blade coating. The ink composition of this type contains as an ink solvent a volatile organic solvent, such as methyl ethyl ketone, toluene or xylene, which often includes a thermosetting resin or a UV-curable resin. The ink composition is easily cured by applying heat thereto or irradiating ultraviolet rays, and thus finds industrial applications in most cases, but hardly used in offices and homes. For example, the ink composition is used for printing manufacturing dates on the bottoms of cans of soft drinks.
In recent years, however, the above-described ink composition has been developed so as to exhibit excellent water fastness and lightfastness, so that the ink can be suitably used for printing by ink jet printers, which are usually used in offices and homes. To this end, the ink is required to exhibit optimum characteristics. More specifically, the ink should be free from sedimentation or coagulation even if it is not used for a long period of time, that is, should exhibit high storage stability. Further, the ink jet printer using the ink should not suffer from clogging of jet nozzles formed at the operating end of the printhead or ink channels formed in the printer, and is able to produce high quality print images.
The pigment-containing ink composition thus developed has improved water-fastness and light-fastness. However, the ink is still likely to clog the jet nozzles or ink channels of the ink jet printer, and exhibits relatively low stability after long-term storage, as compared with the dye-containing ink composition as described above. To solve these problems, the size of particles of the pigment, which greatly affects the quality of printed images, should be appropriately controlled, for dispersion in the ink solvent. If the median particle size or average particle size of the pigment particles dispersed in the ink solvent is relatively small, the ink does not incur clogging of the jet nozzles and/or ink channels of the printer, and exhibits excellent stability after long-term storage. If the particle size is too small, however, excessive blurring of printed images may result.
If the median size or average particle size of the pigment particles is relatively large, on the other hand, clogging of the jet nozzles and/or ink channels of the ink jet printer is likely to occur, and the stability of the ink is deteriorated when it is stored for a long period of time. Further, the ink having a relatively large pigment particle size cannot produce high-quality print images, due to a variation in colors, though the printed images are free from blurring. If bulky pigment particles are present in the ink composition, clogging of the jet nozzles and ink channels of the ink jet printer is likely to occur, and the long-term storage stability of the ink is deteriorated, even if the median or average particle size of the pigment particles is suitably controlled to a desired value.
If the pigment having a relatively low tinting strength is used in the ink composition, resulting printed images do not have a sufficiently high OD (optical density) level, that is, relatively low-quality print images may result, irrespective of the median or average particle size of the pigment. In this connection, printed images are considered to be of high quality if the OD level is not less than 1.2.
The above-indicated median or average particle size, presence of bulky particles, and tinting strength of the pigment particles dispersed in the ink solvent are greatly influenced by characteristics of the pigment itself. In other words, the pigment may be suitably selected to achieve desired median or average particle size and tinting strength, while avoiding inclusion of the bulky particles.